In the underground construction industry, there are instances where elevation changes in pipes, boreholes, and tunnels must be measured to a high tolerance where specific slopes and grades are required to be implemented in the construction. Post-construction pipe slope irregularities may be present for reasons including faulty design, improper subgrade preparation, inadequate backfill practices, or differential settlements. While trenchless boring technologies such as Micro-tunneling and Horizontal Directional Drilling can provide precision over long distances, trenchless pipe installations can still experience out-of-tolerance grades due to improper installation, soil conditions, etc.
Exact grade surveys of newly installed and existing pipe are difficult to obtain for in situ pipe. Deficiencies such as offset joints and severe pipe sags can be visually detected with CCTV optics. However, more gradual out-of-tolerance discrepancies are harder to detect with optical tools or with existing survey technologies. Optical tools also cannot provide grade surveys around curves, corners or submerged conditions. In micro-tunneling, the conformance of slopes and grades to specification can be determined by laser spotting lengthwise from a jacking shaft. But heat refraction due to temperature changes in the tunnel may result in inaccuracies, and laser tools cannot be used around curves.